I once had a fixure light that in my kitchen. It was
instant-start. The tubes had one pin on each end. They
started with cold cathodes, which required a lot of voltage.
Because of that voltage, each pin was a sort of interlock to
break the primary circuit if the bulb was removed.

Rapid-start and preheat tubes pretty well replaced them
because they were would stand a lot more start cycles. I've
never seen a schematic for a magnetic rapid-start two-tube
ballast where removing tubes broke the primary circuit.

I just checked my kitchen light. Without tube 2, both ends of
tube 1 glow. Without tube 1, tube 2 will light up. So either
tube can be removed without breaking the primary circuit.
With no tubes, I read 11.2 ohms between hot and neutral. The
primary circuit is not open.

Stretch wrote:

When you remove both of the 4' or 8' tubes on one ballast, it opens the
circuit of the power going to the ballast. Check the fixture ends
(bulb holders) and you will see what I mean. Therefore there is no
power applied to the ballast and it uses ZERO electricity. SHEESH!!!

Stretch